Arrays comprising a plurality of laser diodes are well known in the art. In one application of laser diode arrays, individual diodes can be modulated so as to expose an IR sensitive printing member on a drum. In one known application, the drum is part of a thermal printer as described for example in U.S. Pat. Nos. 5,109,460 and 5,168,288 assigned to Eastman Kodak Company (Kodak) of Rochester, N.Y., U.S. In a second application, the drum may be a part of digital printing press as described for example in U.S. Pat. Nos. 5,357,617 and 5,385,092 assigned to Presstek Inc. of New Hampshire, U.S. In a third application the drum may be a drum of a computer to plate image setter.
Generally speaking, two types of IR diode lasers imaging apparatus are known in the art. In one type, described in the above mentioned patents assigned to Presstek Inc., the light emitted by each laser diode is focused by a corresponding focusing lens. Thus, a large number of lenses are required, whereby the complexity and the cost of the imaging apparatus increase.
In the second type of imaging apparatus, described in the above mentioned patents assigned to Kodak and schematically illustrated in FIG. 1 to which reference is now made, the thermal printer 1 includes a movable imaging apparatus 10 moving in the direction indicated by arrows 2 to affect line by line scanning on a drum 11 rotating about a longitudinal axis as indicated by arrow 4.
The movable imaging apparatus 10 comprises an array of IR laser diodes 12 of which five, referenced 12A–12E, are shown in FIG. 1. Each laser diode 12 is attached to a corresponding optical fiber 13A–13E in a pigtail type attachment, the light emitting ends of the plurality of fiber optics are aligned at 14.
In this type, the light from all IR laser diodes 12 is focused onto the drum 11 by a single optical assembly 15. The optical assembly 15 comprises a stationary lens assembly 16 and a movable focusing lens or lens assembly 17. In FIG. 1 an exemplary light path 18C is shown for the light emitted by laser diode 12C to affect exposure of the medium mounted on drum 11 at exposure spot 19C.
One drawback of IR laser diodes is that in order to obtain the output power required to expose the IR sensitive medium, fiber optics with a large diameter, typically 100 microns, and a large numerical aperture, typically larger than 0.2, are required. Moreover, in order to meet quality requirements of the exposed image, the focusing lens images the output of the fiber optics with a demagnification ratio of 3, thus leading to a numerical aperture of 0.6 towards the image plane.
Since the numerical aperture of the focusing lens is high, an autofocusing mechanism is designed to compensate for changes in the distance between the surface of the printing member and the aligned light emitting end 14 of the fiber optics 13. This autofocusing compensation mechanism includes the movable lens or lens assembly 17 which is movable between stationary lens assembly 16 and the drum 11 as indicated by arrow 6.
In the illustrated example, lens 17 moves from its position 17 to its position 17′ as indicated by arrow 6 so as to change the optical path from 18 to 18′ in order to expose the light sensitive medium in exposure spot 19C′ thus compensating for the movement of the medium on the drum 11 as indicated by location 11′ of the drum.
A drawback autofocusing optical assemblies, in particular ones which provide an accuracy of the exposed spot in terms of location and spot size on the order of microns is their cost and complexity and the fact that they are prone to mechanical failures.
A lens assembly known in the art which replaces autofocus lens assemblies is shown in FIG. 2 to which reference is now made. FIG. 2 illustrated a system similar to that of FIG. 1 except that it includes a stationary lens assembly 25 instead of the autofocus lens assembly 15.
In a system with a prior art stationary lens assembly, a change in the distance between the distance of the printing member on drum 11, schematically illustrated by the dashed drum 11′, and the aligned edge 14, results in a change in the location of the corresponding exposure spots from 19A and 19E to 19A′ and 19E′, respectively. As illustrated in exaggeration for illustration purposes in FIG. 2, the lateral distance between exposure spots 19A′ and 19E′ is larger than the lateral distance between exposure spots 19A and 19E, i.e., the position accuracy of the exposure spot on the drum 24 is adversely affected by changes in distance between the printing member and the aligned edge of the optical fibers 14.
Printing members, typically in the form of waterless printing plates, for use with lithographic printing presses and components therefor, commonly have an oleophilic (ink attractive) substrate layer that is usually either aluminum or polyester; an intermediate infra-red radiation absorbing layer that could be carbon or other infra-red radiation absorbing material, such as Nigrosine® dissolved or suspended in a binder resin, or a metal or metal oxide film such as titanium oxide sputtered onto polyester as the infra-red absorbing layer; and an oleophobic (ink abhesive) polysiloxane top coating layer.
These plates are imaged, typically by ablation with an infra-red laser, such that an image is placed on the substrate layer, that is oleophilic, to attract and retain the ink. The ablation process completely destroys the intermediate infra-red absorbing layer, and causes the polysiloxane coating layer to detach from the plate as well. Complete removal of the polysiloxane top layer affected by the ablation commonly involves additional cleaning. This additional cleaning is typically performed with a dry cloth or with a liquid, that may have a solvent effect. The cleaning process results in the complete removal of both the top polysiloxane layer and the intermediate infra-red radiation absorbing layer, leaving bare portions of the now imaged substrate layer.
When waterless offset printing is desired, a printing plate is mounted on a drum or the like and contacted with one or more forme rollers onto which a thin layer of waterless ink has been deposited. Where there is still silicone on the background areas of the plate, the ink is retained on the inking roller as it will not transfer to the plate surface, which has a very low surface energy and is termed abhesive and is oleophobic. The bare portions of the substrate provide an oleophilic surface and ink transfers from the ink roller onto the bare portions of this surface, such that the inked image may be transferred by an offset blanket (cylinder) onto printing media, such as paper.
These plates exhibit several drawbacks. Initially, the complete removal of the ablated top oleophobic coating and the infra-red radiation absorbing intermediate layers, which together may be several microns thick, results in a physical difference in height above the substrate layer. The distance between the unimaged remaining top coating layer and of the depressed imaged substrate layer, gives the plate an intaglio nature. Because this distance is large, transfer of the ink from this plate requires increased pressure of the forme rollers with respect to the ink surface, compared to that for planographic plates, to ensure that the ink reaches the depressed image surface. This in turn reduces the plate run life, because the increased pressure creates additional wear on the plate, shortening its usable life. This increased pressure also increases the chances of physical damage to the plate during running, such that a printing run may have to be prematurely terminated due to a damaged plate. In addition, because the surface of the image deeply depressed from the polysiloxane surface layer of the plate, the portions of the substrate to be imaged are set back from the inking roller (ink transferring source) at a distance such that there is a reduction in the ease of initial inking up of the plate. This increases the inking or coloring time for the plate and blanket cylinders, and subsequently, the number of copies necessary to be run before fully inked up copies start appearing.
Another drawback with these plates, that effects their imaging quality, is associated with their cleaning. These plates originally were hand cleaned, and as such, permitted the operator a great deal of involvement in ensuring good results by visually selecting imaged areas to be cleaned while leaving the unimaged areas not to be cleaned, and consequently, cleaning only those areas that required cleaning. Also, where the plates were ablated with high energy, it was possible to blast away the largest part of the top layer and the ablatable intermediate layer, so that any remaining loose material involved minimal wiping.
However, where the ablation energy is relatively low, it is necessary to clean these plates thoroughly. This is typically done automatically. However, automatic cleaning subjects unimaged areas to unnecessary cleaning, that can damage the background (remaining plate layers), and thus, reduce plate life. Cleaning also has to reach the depressed areas of the substrate, thus increasing cleaning difficulties.
A further difficulty with the plates is their lack of sensitivity to the infra-red radiation. This poor sensitivity results in using multiple high energy lasers in an array, that adds to printing costs.